3d Printing with Tinyg ( rotational axis A trouble )

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  • February 25, 2018 at 9:13 pm #10873
    cmcgrath5035
    Moderator

    It is very interesting what you are doing.
    With regards to yawsticks observation that pump motion seems to be slow-fast-slow, that is likely due to jerk – the concept of jerk is to manage momentum as smoothly as possible; a move form stop accelerates slowly at first, then acceleration increases to achieve the set max velocity, then the motion decelerates on a similar curve. You could try to modify $ajm to see how that affects results.

    Some comments relative to your manipulation of stepper A drive current to induce waves. The on board pots adjust the amount of power delivered to the stepper, not the voltage. The stepper driver monitors the current as it ramps up in the motor windings and cut off pulses of full voltage when the set current (by the pot) is achieved. The current thru an inductor (the winding) changes based on the drive impedance of the stepper driver and the inductance of the winding. I would expect operating, as you are, right on the threshold of missing steps, would produce rather random results and be subject to thermal issues, since the stepper will tend to miss more steps when driven by marginal currents at high temperatures. That does not explain the repeatability of your erratic results.

    February 26, 2018 at 12:19 pm #10874
    whitecloud
    Member

    Thanks cmcgarth5035 – The character of the extrusion (slow-fast-slow) is mechanical, and not a result of jerk settings. It is a peristaltic pump that squeezes a rubber tube to meter the ceramic slurry. Each cycle has beginning and end – one point of interest, with this mechanism, the amount of torque required of the stepper builds and then drops at the end of each cycle.

    And thanks for your clarification on the board pots. I think I am missing some steps, so perhaps the power is a little low. It looks like a way forward would be to conduct some tuning/tests with the pot on the A axis. Still doesn’t explain the repeatability of the erratic results, but it is a start.

    February 27, 2018 at 7:08 am #10875
    cmcgrath5035
    Moderator

    I agree with your comment “Still doesn’t explain the repeatability of the erratic results, but it is a start.”

    It might be interesting to experiment with two different F speeds for your X,Y linear motion, point 1 to point 2 at F1, point 2 to point 3 at F2, with a pump cycle between each.

    February 27, 2018 at 8:34 am #10876
    whitecloud
    Member

    thanks cmcgarth – That sounds like a good test. I think I can run it today. If you still think that the jerk may need to be addressed, would you suggest that I increase or decrease the value?

    I ran a test yesterday – without slip – ran single curves from the bottom, middle, and top of the vases from the video I previously posted. I timed extruder rotations per 30 seconds and they were all identical – with the current feedrate, approximately 11 revolutions.

      To further confuse, one thing I noticed was that even without a load there was buzzing from the extruder stepper – the pot was initially set at 12 o’clock, and an increase to 1 and then 2 increased the volume of the buzz. Turning the pot down to 11 and then 10 reduced the volume, but the buzz remained. The stepper did not buzz when it was new.

    Amazon link to the stepper that is currently on the extruder:

    I switched to this larger stepper some time ago when the original stepper begain clicking. Thinking it may be underpowered, I swapped it for the larger one above. clicking still occurs after the machine has been running for an extended periods. Any thoughts on the cause, or if it might be related?

    I’ll be swapping back to the previous stepper today, and I think I can run the suggested test with both steppers over the next 2 days. Link to the original stepper which I’ll be switching back to today. It is identical to those on all of the other axis:

    https://www.inventables.com/technologies/stepper-motor-nema-17

    February 27, 2018 at 10:09 am #10877
    yawstick
    Member

    I would go along with that suggestion… in the extreme make your pump speed constant and vary only the x,y, speed… to produce the irregular effect… otherwise you have about exceeded my ability to contribute other than to observe… it is an interesting project. I am curious about the pump stroke… does the stepper for A axis come to a stop in the cycle or just slow down. If so it seems the jerk could be a factor unless you’ve overridden it somehow. If not it would seem worth experimenting with… as if it works as described would help create the fat in center deposit without having to create it in the program.
    You could also experiment with making A, a fucntion of distance traveled in X,Y. and introduce some jitter into that to avoid or reduce the condition of the droplets stacked exactly on top of one another.
    I realize that’s not addressing whats causing your issue… but any fix is a good fix 😉

    February 27, 2018 at 12:48 pm #10878
    yawstick
    Member

    I started the post above before I had seen whitecloud’s most recent post… thought I could edit it but it appears not. Added this so it would be more clear that I was responding to cmcgrath5035 suggestions. I also wonder if you’ve had this issue from the beginning or it developed along the way.

    February 27, 2018 at 12:55 pm #10879
    whitecloud
    Member

    Thanks at yawstick – The extrusion values are currently defined by distance traveled for each subsequent point (every 5mm along a curve or single layer of a vase). I like the idea of jitter in the extrusion value. I will try that at some point – after I get the darn thing behaving.

    The A stepper only stops between curves. As I understand, the tinyG will evaluate it’s path and the related speed/jerk/acceleration every time it is given a new point to travel to. The extruder stepper only ‘stops’ or pauses at the end of a completed curve before traveling to the next curve and starting again.

    February 27, 2018 at 1:09 pm #10880
    whitecloud
    Member

    thanks again yawstick – trouble seems to have developed recently – in the past I had some irregularity that was easily managed with the onboard pot or addressing material consistency. My ceramic slurry is very consistant now.

    February 27, 2018 at 4:16 pm #10882
    yawstick
    Member

    I originally envisioned your extruder pump as a screw in a barrel type pump. I had to google yours to see what you meant. I would think that type of pump would naturally have a bit of a pulse to the output when running at a continuous speed. Do you have a link to it? How many steps required for a full revolution?

    Is it possible to remap the motor outputs so that you could swap A & Z to different driver chips.

    Have you tried just making just one vase form to see if it still happens in the same place in Z?

    February 27, 2018 at 6:44 pm #10885
    cmcgrath5035
    Moderator

    Clicking Steppers usually is an indication of overload(inadequate torque available) which could be caused by ot setting or simply too much torque demand by the pump.
    You could try to change the microstepping setting from 8 to 4 or two, that might help.
    But, as a general statement, using torque reduction to force missed steps should be expected to result in clicking.

    March 8, 2018 at 8:17 am #10897
    yawstick
    Member

    Curious if you came up with anything on this?

    March 8, 2018 at 8:56 am #10898
    whitecloud
    Member

    I have conducted a few tests – to no prevail:

    3/1/18 printed a series of circles utilizing the same gcode but varying the feedrate (7 single layer circles that are approximately 6″ in diameter). The results is that the extrusion rate remains consistent regardless of feedrate (100 – 1500) (250-1000 is my normal operating range).

    3/3/18 – Reduced Micro stepping on the extruder while printing the same file as above. Tested at 4 and 1 microsteps. No visible change in extrusion rate – was able to reduce the power to the stepper with the POT and found that there was a great decrease in operating temperature of both the chip and the stepper (I’ve been monitoring this with a laser thermometer – never any higher than 120 f – much cooler with lower microstepping) – the trade off appears to be increase noise.

    3/4/18 ran the tall vase gcode (the problematic one in the video) from my PC instead off the Raspberry Pi – very similar results (3 apparent extrusion rates at the bottom, middle, and top). seem to be at approximately the same points in the code.

    3/5/18 ran tall vase gcode without microstepping on the extruder – again very similar results (3 apparent extrusion rates at the bottom, middle, and top)

    3/4-3/5 – saved and reviewed response from tinyg – no errors detected

    April 10, 2018 at 9:16 am #10956
    whitecloud
    Member

    Still having some trouble. I’ve experimented a bit with adjusting the jerk settings on the A axis.

      If I understand, reducing the jerk would create smoother acceleration

    I have reduced the value by half and then quartered – it hasn’t seemed to help. I have not tried increasing the jerk value. Any advice on tuning the A axis?

    Also – could the erratic steps be heat related? Thinking about heatsinks and/or a better fan. I currently have a 12v 40mm fan blowing directly down on the board from just a few inches away – I could mount a small blower directly toward the chip on the A driver.

    April 11, 2018 at 7:25 am #10957
    cmcgrath5035
    Moderator

    This remains an interesting thread, but difficult to comment on without hands on experience.
    Since you are tweaking the current drive to your A stepper to achieve the desired effect, I would say for sure that temperature will have some effect on the results. Can better temperature control make for more predictable results? Maybe, maybe not.
    Your existing cooling setup sounds like one that is usually adequate in a digital world. By tweaking current, you are really entering a Analog world where minute variations of temperature, voltage, electrical noise, even sunspot activity could affect the repeatability.

    Here is a thought experiment, which you may be able translate into a real experiment with conditions appropriate to your setup.

    Put a color dot on the pump drive mechanism so you can watch physical rotations.

    Create a Gcode Command that moves your extruder 20mm in a straight line while making one rotation of the extruder driver (the A axis)

    Send that same command three times, manually from CoolTerm, with an arbitrary pause between.

    You will have a 60mm line of extruded material.

    Now move the extruder so that it can make a parallel extruded line, and send a Gcode command that will move 60mm with three rotations of the extruder.

    In both cases, did the pump rotate three full revolutions?

    Compare results to understand the effect of segmenting the process.

    Keep in mind that tinyG, specifically the planner, would look ahead in a stream of three discrete move commands and implement what would look like a single move, not stopping/starting the extruder.

    I think you understand what I am suggesting.

    • This reply was modified 6 years, 5 months ago by cmcgrath5035.
    October 6, 2018 at 4:46 am #11139
    Pia Tinoco
    Member

    It is interesting to read your blog post and I am going to share it with my friends.

    Herbew Fentos

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